IN VIOLATION OF THEIR COMPANY OPS MANUAL; B737-700 FLT CREW USES THEIR FMS FOR AN NDB APCH TO RWY 12R AT SAT; TX.

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GOT ATIS 150 MI FROM SAT. FIELD VFR. ILS/VISUAL TO RWY 12R. DSCNT ROUTINE. HANDED OFF TO APCH AND WAS CLRED TO ALAMO (OM) FOR NDB. I HAD THE ILS ALREADY PROGRAMMED IN THE FMC AND WENT DIRECT TO ALAMO. WE THEN SELECTED NDB RWY 12R FROM THE ARR/APCH PAGE AND BRIEFED THE NDB APCH. THE ILS APPEARED TO BE WORKING AND USED IT FOR BACK-UP. DSNDED BELOW THE BROKEN LAYER; PICKED UP THE FIELD AND PROCEEDED VFR TO UNEVENTFUL LNDG. I THOUGHT WE COULD USE THE FMS FOR NDB APCHS. WE USE IT FOR MISSED APCHS AND HOLDING AND IT IS IN THE DATABASE. I OBVIOUSLY MISSED THAT SMALL PARAGRAPH IN THE OPS MANUAL. OOPS! SUPPLEMENTAL INFO FROM ACN 616427: THE ILS DATA ALL SEEMED TO BE WORKING NORMALLY. AFTER LNDG; WE DISCUSSED THE ISSUE OF WHETHER OR NOT WE WERE ACTUALLY LEGAL TO SHOOT THAT 'NDB' APCH. MY QUESTION IS; WHY WOULD APCH ASSIGN AN NDB UNDER DETERIORATING WX CONDITIONS WHEN THE ILS WAS OBVIOUSLY FUNCTIONAL? THE ILS TO RWY 3 WAS AVAILABLE; BUT SINCE WE WERE ALREADY OVER AN HR LATE; ADDING 5 MORE MINS TO THE FLT WAS NOT A VERY PALATABLE CHOICE.